The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device that drives liquid crystal molecules by applying voltage between an upper electrode layer and a lower electrode layer that are formed on the same substrate via an insulating layer, wherein one of the upper electrode layer and the lower electrode layer is a common electrode layer and the other is a pixel electrode layer, and a plurality of openings are formed in the upper electrode layer and arranged parallel to each other so that an electric field is passed therethrough.
In regard to a display type of an existing liquid crystal display device, a TN (twisted nematic) type display device has been widely used. However, the TN type display device has a limited viewing angle due to its display principle. A horizontal electric field type display device has been known as a solution for the above problem, in which a pixel electrode and a common electrode are formed on the same substrate, voltage is applied between the pixel electrode and the common electrode to thereby generate an electric field that is substantially parallel to the substrate, and liquid crystal molecules are thus driven mainly within a plane parallel to the surface of the substrate.
In the horizontal electric field type display device, an IPS (in plane switching) mode display device and an FFS (fringe field switching) mode display device are known. In the IPS mode display device, a comb-shaped pixel electrode and a comb-shaped common electrode are arranged in combination. In the FFS mode display device, an upper electrode layer and a lower electrode layer are formed via an insulating layer, in which one of the upper electrode layer and the lower electrode layer is used as a common electrode layer and the other is used as a pixel electrode layer, and then openings, such as slits, are formed in the upper electrode layer so that an electric field is passed therethrough.
It is conceivable that the relationship between the transmittance ratio of the horizontal electric field type liquid crystal display device and the driving voltage thereof is different from that of the TN mode display device, or the like. There have been some proposals for improvement of the transmittance ratio of the horizontal electric field type display device.
Japanese Unexamined Patent Application Publication No. 11-202356, for example, describes an IPS mode liquid crystal display device, that has a high transmittance ratio and a high aperture ratio, in which the strip width of each pixel electrode is formed narrower than the distance between adjacent strips of an opposite electrode, the distance between the strip of the pixel electrode and the strip of the corresponding opposite electrode is smaller than a cell gap, the widths of the strips of the opposite electrode and the widths of the strips of the pixel electrode are formed so that liquid crystal molecules present on these electrodes are movable owing to parabolic electric field generated therebetween.
As described above, JP-A-11-202356 describes display quality in the case of the IPS mode display device, but it does not always apply to display quality in the case of the FFS mode display device. That is, the FFS mode display device has a slit-shaped or a comb-shaped electrode structure, which is only used for an electrode disposed on the outermost surface side among the pixel electrode layer and the common electrode layer, and the electrode having the slit-shaped or comb-shaped electrode structure in the outermost surface layer is formed so that electrode portions having the same electric potentials are arranged with openings formed therebetween. This structure differs from that of the IPS mode display device. Thus, the positional relationship between the openings and the electrode portions in the electrode disposed on the outermost surface side, that is, the relationship between the width L of the conductive portion of each electrode portion and display quality and the relationship between the short side width S of each opening and display quality are different from those of the IPS mode display device.